The present invention relates to the field of compatible AM stereophonic systems of transmitting and receiving, and more particularly, to a system wherein one program signal may be derived from the upper sideband signals of the received transmission and the other program signal from the lower sideband signals.
A system known in the art (U.S. Pat. No. 3,908,090) for providing a similar transmitted signal also derives sum and difference signals from two program signal sources (L and L) and shifts them to provide a 90.degree. phase difference with respect to each other. The goal of this system is to transmit information derived from one program signal on one set of sidebands and the second program signal information on the other set of sidebands. However, this system utilizes a two-term series (first and second order sidebands) and operates on only the second order term to provide the desired output spectrum. The carrier is amplitude modulated with the sum signal (1+L+R). The two program signals (unshifted in phase) are individually coupled to frequency doublers and the double frequency signals are combined substractively to provide a second difference signal of the form [L(2.omega.)-R(2.omega.)]. This latter signal is amplified in a variable gain amplifier. The first difference signal (L-R) is coupled to a rectifier, and the variable gain amplifier is gain-controlled by the rectifier output signal. The rectifier/amplifier combination is termed a "level squarer." Thus the second order sideband level is controlled by the level of the first order sideband, but the higher order sidebands are unaffected. The variable gain amplifier output signal is added to the first difference signal and a carrier frequency signal is phase modulated by the combined signal. Since the modulator produces relatively small amounts of phase modulation, the phase modulated signal is frequency multiplied, then shifted in frequency to the carrier frequency of the transmitter. With this form of transmitted signal, it is possible to produce stereophonic reception of a sort by tuning one monophonic receiver to a frequency approximately one-fourth channel width lower than the carrier frequency and a second receiver to a frequency correspondingly higher than the carrier frequency.
This system of transmitting stereophonic information substantially shifts first and second order sidebands, but does nothing for the higher order sidebands which can produce perceptible distortion components in the received stereophonic signal. When it is desired to provide an AM stereo signal with left information on one set of sidebands and right information on the other set, it is important to provide undistorted information on each sideband.